The vascular system of human renal clear cell carcinoma was studied using light microscopy of silicon rubber-injected specimens and scanning electron microscopy of conventionally prepared tissue and vascular corrosion casts. The system was found to exhibit the following features: (1) a well developed superficial vascular coat showing different pattern on the anterior and on the posterior side of the tumour, (2) an internal vascular network composed of altered and displaced preexisting vessels, numerous newly formed ones and those recruited from adjacent structures, (3) quantitative prevalence of dilated veins and distended capillaries, (4) a remarkable proliferative reaction of stellate veins, (5) characteristic features of the intratumour vasculature in the form of avascular nodules surrounded by basket-like capillary plexuses and separated by well vascularized "septa", (6) a relatively less dense vascularization of central tumour areas, frequently exhibiting necrotic foci, and the highest density of vessels in areas close to the superficial vascular coat, and (7) morphological evidence for a continuous remodelling of the tumour vasculature. The observed patterns of the vascular system seem to provide a pathway for further tumour expansion.